It's a plan that already exists. Subito.it (based on osclass, at least initially) implements it, as does ToGoodToGo. It could be created using the site administrator's account as an intermediate (paypal?) account. The user who purchases sends the money to the administrator. If the buyer confirms correct receipt of the package, clicks on a link that allows payment from the administrator to the seller, otherwise he opens a dispute and the administrator verifies the proof of shipment or product and decides who to protect. It is a function that mainly protects buyers from fraud and I believe it will also spread to other sites and apps.